Systems designers and integrators are familiar with the
typical sensor platform architecture that includes an amplifier/signal
conditioner with an active analog filter (requiring an external, stable and
regulated power supply), a multi-meter (display) and data storage. In automated
systems use, this sensor architecture also requires instrumentation to convert
output into digital numbers as well as specific programming to display,
manipulate and use the data collected by the sensor. †It is this device connection that often
introduces the most complexity into the system design for engineers depending
on the application.
USB Interface Option
Using a USB connection-the same one we're all familiar with
on our PC peripherals-all configuration is done in the background of the device.
Industrial level USB connections bring this familiarity to complicated sensor
systems so they can all be connected in one way, eliminating potential for
misconnection along the sensor chain in an industrial system.
Industrial USB connections bring familiarity to complicated sensor systems so they will only connect one way, eliminating any potential for misconnection in the sensor chain. Source: FUTEK.
Any computer or other USB-enabled readout device loaded with
interface software will allow full sensor system interface. An additional benefit
of using a USB sensor chain is that it utilizes the computer's power supply.
Depending on the sensor's power requirements, most systems will not need an
additional power supply.
The USB interface also removes the need for an amplifier
(signal conditioner), analog filter, power supply, and multi-meter.
Other advantages of USB, besides the cost, include the high
resolution digital output, an integrated SINC digital filter to achieve smooth
low pass filter, the ability to increase the sampling rate for high speed applications,
and capability to store calibration values inside the onboard, non-volatile
memory (which is required to apply the real-time calculation to account for
non-linearity). As a† result, the TEDS
chip typically used to store these and other configurations is no longer needed.
High quality USB modules also have long-term stability and a
low temperature coefficient compared to analog amplifiers due to the elimination
of the analogue circuits. These modules are also not as sensitive as analog
amplifiers to noise because they use digital processing. They also have a higher
Common Mode Rejection Ratio (CMRR) and Power Supply Rejection Ratio (PSRR)
compared to analog amplifiers. CMRR and PSRR directly cause the signal-to-noise
ratio (SNR), which defines the level of output noise.
In the best conditions, most analog amplifiers have more
than 1 millivolt internal noise, while the internal noise of high quality USB
modules is approximately 1 microvolt.
A high speed, bi-directional USB link gives system designers
the ability to communicate with sensors via PC-based software and
monitor/control the functionality of features such as tare, peak, valley and
selectable averaging, without having to manually change hardware settings. This
type of system verifies that the sensor is functioning properly, so it removes
any risk of not installing the sensors correctly.
Another benefit of USB technology is that, in addition to
its ability to handle any brand, type or model of sensor, it can also handle
multiple sensors. One of the biggest headaches for sensor design engineers is
that every different sensor typically requires a different controller. Using
the USB approach, most systems can use one platform. This is true for all standard
sensors, however, some unique sensors may require special USB connectors.
A USB connection
allows any sensor with analog output to interface cost effectively with digital
devices. For example, a 100 gram load cell can be interfaced with USB modules
to resolve at 1 milligram resolution-and potentially even 100 microgram
resolution (0.1 milligram)-for less than $3,000. A similar conventional sensor
system would require a large package of precision instrumentation costing up to
illustrate the cost savings benefits of USB sensors, consider this industrial
application example: In the process of automating hydraulic or pneumatic
presses for feedback control, traditionally one load cell is used for heavy
duty presses with a few million pounds of load capacity. This typically requires
a large, expensive custom sensor. Using USB technology, multiple standard,
off-the-shelf load cells with in-line external USB interfaces could be
interfaced using a hub. This approach not only simplifies installation, but
minimizes total cost. It also allows the operator to check the uniform loading
of a large platform while monitoring the output of each sensor individually. Two
added benefits are easier troubleshooting plus the ability for the system to
remain operational if a sensor is removed for repair or replacement.
The USB solution works for most sensors, including older
systems still in the field. These sensors can be updated with a USB interface and
still use the same sensor configuration. Using a hub, up to five levels of
sensors can cascade to a chip with a single USB port.
Beyond all of the cost saving benefits, the bottom line for
any sensor application is validation, i.e., measuring an important area of a
process or system. This can become a major issue when sensors are recalibrated.
For an ISO 9000 regulated company, any shift in the sensor control system that was not identified would
mean that all parts would be subject to recall because they were not made
exactly to spec.